There are lots of cool features and improvements in Windows 8.1 so I prepared a one pager for Windows 8.1 resources toolkit that includes all links & resources in a one place.
Windows 8.1 Pro – You can install Windows 8.1 Preview using the ISO files (.iso) that are provided on this page.
Bing Maps BETA SDK for Windows 8.1 Store Apps– Binaries & programming ref for the Bing Maps Win8.1 Control
Player Framework – an open source component of the Microsoft Media Platform
Live SDK 5.4– The Live SDK provides a set of controls and APIs that enable applications to integrate Single Sign On
Multilingual App Toolkit for Visual Studio 2013 – Localize your Windows 8 app with translation support
Remote Tools for Visual Studio 2013 Preview –Remote debugging on computers that don’t have VS installed.
Upgrading Windows 8 Apps to Windows 8.1 – Breaking changes documentation from release preview
Windows 8.1 Preview: New APIs and features for developers – What’s new for developers in Win8.1 Preview
Developer forums – Developer forums for Windows Store apps covering designing, developing, and selling apps.
Building Windows 8 blog (B8) – An inside look at how, what, and why different features of Win8 are being built.
Windows 8 app developer blog (D8) – Explores best practices for coding and designing Windows Store apps
Build 2013 – Windows 8.1 Sessions – Windows 8.1 Keynote and all build session videos